Edited question : Is there any syntax that allow f...

# announcements

[ylemoigne]

Edited question : Is there any syntax that allow for a premature return in when case block ? If a such thing is possible, what it look like ? (Original question : Hello, I have difficulties to find the right syntax (jump label ?) to return from a when clause) Eg :

fun main() {
    val myShortCutCondition = true
    val s = when(2){
        1 -> {
            if(myShortCutCondition){
                // return "shortcut" // for `s` value
            }
            
            // longCode
            
            "1"
        }
        2 -> "2"
        else -> ""
    }
}

[Mike]

Use else for "1" return value. Ideally, don't want multiple return in your code, so use the fact that if is an expression.

But your when will always result in "2" as you're doing a when on the value 2.

[ylemoigne]

The fixed 2 is just for exemple sake. I don't want to create another nest level. So I do want multiple return from the 1->{ ...block... }.

[ESchouten]

Easiest would be to warp it in an else statement, indeed. Alternatively you could move the code to a separate function, which would be advisable for // longCode

[ylemoigne]

Yes moving the code in another function is a way. But to be clear, I'm curious about syntax of jump label, the return@somethine is this particular case. So I should rephrase the question : Is there any syntax that allow for a premature return in when case block ? If a such thing is possible, what it look like ?

[ESchouten]

I see, not that I am aware of, though I would be interested to know as well

[Barry Dempsey]

I've just used Playground and this works if you want to try it

Untitled

[ylemoigne]

Thanks you but it's not the same case. Here the return return from the function. If I was willing to create another function to return from when, I would write :

fun main() {
    val myShortCutCondition = true
    val s = when(2){
        1 -> inCaseOne(myShortCutCondition)
        2 -> "2"
        else -> ""
    }
}

fun inCaseOne(myShortcutCondition:Boolean):String {
    if(myShortcutCondition){
        return "shortcut"
    }
    
    // longcode
    
    return "1"
}

[ESchouten]

@Barry Dempsey the idea is to define the return value of the when statement instead of returning the function it is in

You’d expect something like return@when

[Kristoffer Andersen]

@ylemoigne You could lift the when into a local function. Now return regains it's short-circuiting power:

fun main() {
    val myShortCutCondition = true

    fun whenStmt(): String {
        return when(1){
            1 -> {
                if(myShortCutCondition){
                    return "shortcut"
                }
                // longCode
                "1"
            }
            2 -> "2"
            else -> ""
        }
    }

    val s = whenStmt()
    println(s)
}

[ylemoigne]

Nice tricks I did't think of. Thanks for the suggestion :). (Drawback, it create another nested level. But still more readable than long if .. else in when case).

[Kristoffer Andersen]

What we really want is something like call with current continuation or call/cc as it's often known:

fun main() {
    val myShortCutCondition = true

    val s = call/cc { k ->
        k(when(1) {
            1 -> {
                if (myShortCutCondition) {
                    k("shortcut")
                }
                //longCode
                "1"
            }
            2 -> "2"
            else -> ""
        })
    }

    println(s)
}

[Kristoffer Andersen]

call/cc expects a function that takes "the rest of the computation", known as a continuation, as an argument here named k, which you can invoke to resume the computation at the point of calling call/cc , returning the value supplied to the continuation. But it's not in Kotlin as far as I can tell 🙂

[Kristoffer Andersen]

The pseudo-code I just posted is a generalization of the version with a local function declaration.

[Mike]

What you really want is full pattern matching, then. Something like

when(value) {
   1 && myShortCutCondition -> "shortcut"
   1 -> "1"
   2 -> "2"
   else -> ""
}

But that's not available, and it sounds like the Kotlin team is reluctant to add it. Perhaps as Java adds more pattern matching to switch, we'll see more power in when.

You could 'inline' value into each of the condition steps, but that does create duplication of code.

[ylemoigne]

It seems I would like that because I made a very simple example do illustrate the question. But, what I would really want, is return@when value .

[Mike]

Personally, I don't like embedded returns as I find it harder to read the code, but each team has their own preferences.

And if the when gets too long, then I think it's best to extract it to a function with a meaningful name. If you can't come up with a meaningful name, then it's likely doing too much anyway.

[ylemoigne]

I understand your preference. There is pro and con. Weigth of each vary between context. That's why I rephrased the question to focus on the syntax part (does it exist, or not). Because, you know, it's internet, you ask for how I do XXX with kotlin ? and there is always some to say yeah, you really shouldn't use kotlin for this, here is how to do it in Haskell : ... And then, that's not what your asked for. And you feel bad because the person wanted to help, but you have to say Hey thanks for your effort but that was not the question which is a bit harsh...

[Chantry Cargill]

I tend to branch to null and then null coalesce.